Thursday, December 06, 2007

Irreducible Complexity

Lately I've run across several different definitions of irreducible complexity [Utterly Stupid Quote of the Day]. Most people seem to think that irreducible complexity is defined as something that cannot evolve but that's not the original definition [IDiot Logic].

Here's what Michael Behe says on page 39 of Darwin's Black Box.

By irreducibly complex I mean a single system composed of several well-matched, interacting parts tat contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly (that is by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional.

Behe then goes on to say that an irreducibly complex system cannot evolve by natural selection.

The definition is not a problem. By this definition there are many irreducibly complex systems in biology. For example, the bacterial flagella is a pretty good example as long as you relax the criteria a little bit. (Some of the minor parts can easily be removed without affecting the overall function.)

The problem is not with the definition, it's with the conclusion. Irreducibly complex systems can easily evolve. All that's required is for the simpler intermediates to have some function other than the one seen in the final completed structure. In the case of the bacterial flagella this simpler function was secretion of large molecules. The flagella evolved from a type III secretion system by just adding a few extra components.

Thus, as Behe says above, it was not produced directly by continuously improving the initial function. Instead, there were several intermediate functions (e.g., secretion) that preceded the shift to the final function we observe today. This is how irreducibly complex systems evolve.

The citric acid cycle is another example of an irreducibly complex system for which there's an easily understood evolutionary pathway. The circular pathway arose when the ends of a forked pathway were joined by evolution of a single enzyme [Defining Irreducible Complexity].

Irreducible complexity is a concept invented by Intelligent Design Creationists. You'd think they would at least make the effort to understand something they created!

11 comments:

Don't forget "scaffolding" (at least the term I used, following from the arch analogy), where a part that is initially unnecessary but beneficial becomes essential as the system evolves. To go back to the bacteria flagellum, there are parts that are absent in some bacteria flagella, present but not necessary in others, and present but necessary in others. So you have some organisms that don't have it, some organisms were it is not an irreducibly complex part, and other organisms where it is an irreducibly complex part. That kind of contradicts Behe's entire argument.

Isn't another flaw in Behe's argument (that his definition of IC is a problem for evolution) that an IC system can be the result of the loss of a part from a previously non-IC system?

For example, an arch consisting of stone blocks shows IC as the removal of any block causes the arch to collapse. However, the arch can be constructed by successive modifications where an initial support is in place. The subsequent removal of the support gives an IC system that has evolved through successive modifications, one of which was the loss of a previously required part.

Loss of gene function (and loss of whole genes) is a well known and evidenced phenomenon due to deleterious mutations. Therefore the loss of a previously required, but subsequently redundant, part gives an IC system that can evolve through successive additions followed by a deletion.

The expression, "irreducible complexity" was invented by creationists, but the concept is an old one. Check the Wikipedia article "Irreducible complexity", where under the heading "Forerunners" there is brief mention made of several writers who used ideas very much like IC, hundreds of years before Behe.

Particularly interesting is the use of something like IC to defend the idea of biological preformation. Preformation seems rather like creationism applied to the origins of individuals, rather than "kinds".

As originally formulated, IC systems are non-controversial. They exist and can be reasonably determined by deletion analyses. The controversy, as Larry notes, is whether a biological IC system is evolvable. ICness refers to the state of the system *as it exists today*. The problem is that many anti-evolutionists accepted Behe's arguments that IC systems couldn't evolve and thus were reliable indicators of (and effectively synonymous with) design. The trouble, as we all know, is that the "show your work" step of the proof never materialized.

But it's not just others that made the mistake of conflating ID with unevolvability...

Behe himself made subsequent new definitions to 'IC' that took into account the pathways of the system in question and in so doing basically defined an IC system as one that is likely to be unevolvable. The trouble was that those were radically different formulations (call them IC-Mark I and IC-Mark II); ones that presupposed a historical route and thus were not independent assessments of the system as it stands today. And he never did demonstrate that any system was "IC" based on these newer criteria. Yet Behe was more than happy use the same term "IC system" interchangeably for all three of his definition classes. Thus he also added to the muddling of the concept. Worse still, there were added terms such as "IC cores" which refer to the "essential" ICness at the core of a complex system such as blood clotting. Thus Behe and other supporters would say that some components of blood clotting which, while essential but clearly evolvable, were not part of the "IC core" and thus didn't detract from his original argument.

Go figure. Some people keep beating dead horses, thinking that they'll eventually get a ride.

"...because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional."

I think Behe is being disingenuous here. He assumes irreducible complexity here and then when he says "by definition" he means "by definition of irreducible complexity". And of course, we know that precursors to these systems are functional even when they are missing a part. So I don't understand why this statement of his is sensible.

However, the arch can be constructed by successive modifications where an initial support is in place.

That is the analogy of biological "scaffolding" I have come to expect, which explains why the key stone only appears to be essential at all times when in reality scaffold and key stone change roles.

under the heading "Forerunners" there is brief mention made of several writers who used ideas very much like IC

Looks like a good article on that point.

Most damning is that Muller's concept of interlocking complexity precedes Behe's in time and scope, which The Panda's Thumb has observed on several occasions. If Behe or any IDer were serious about ID as 'science', they would accept the scientific term. They would also have to show why the verification of Muller's prediction isn't part of evolution of course.

new definitions to 'IC' that took into account the pathways

That was an interesting analysis. I have distinguished between Behe's original deterministic claim ("impossible") and the subsequent probabilistic ("improbable" - but apparently not for his own example of malaria). But the discussion of history shows how much that Behe has come to surrender to science facts and theory.

All, really, as no one questions that evolution is contingent and any one pathway may in fact be a priori improbable in the face of all these possibilities. But the a posteriori likelihood that an evolutionary pathway is used has been found to be indistinguishable from 1. :-P

> Toms: For a second estimate, I have an upper bound for the probability that ID would do it: Strictly less than the probability that natural causes would do it.

In that case we reserve judgment, if based on what we know, we can compute a bound on the probability of nature generating a structure.

But if a good case can be made for a supernatural God (I think it can), then this God designing in the world is not like "unknown being X making Y in some unknown way Z." Design then would become more plausible, being more specific.

Here by the way is Behe's definition of IC in terms of steps: "If a system has to pass through one unselected step on the way to a particular improvement, then in a real evolutionary sense it is encountering irreducibility: two things have to happen (the mutation passing through the unselected step and the mutation that gives a selectable system) before natural selection can kick in again. If it has to pass through three or four unselected steps ... then in an evolutionary sense it is even more irreducibly complex. The focus is off of the 'parts' (whose number may stay the same even while the nature of the parts is changing) and re-directed toward 'steps.' "

"Envisioning IC in terms of selected or unselected steps thus puts the focus on the process of trying to build the system. A big advantage, I think, is that it encourages people to pay attention to details; hopefully it would encourage really detailed scenarios by proponents of Darwinism (ones that might be checked experimentally) and discourage just-so stories that leap over many steps without comment. So with those thoughts in mind, I offer the following tentative 'evolutionary' definition of irreducible complexity:

"An irreducibly complex evolutionary pathway is one that contains one or more unselected steps (that is, one or more necessary-but-unselected mutations). The degree of irreducible complexity is the number of unselected steps in the pathway."

I wrote:> And he never did demonstrate that any system was "IC" based on these newer criteria.

lee_merrill:"That was most of his Black Box book, actually, demonstrating the implausibility of various steps that should be expected on all paths. Thus no need to presume a historical sequence, may it be noted."

That is *not* the case, lee_merrill. Behe's Black Box book only dealt with the first definition of IC: That of a system of interacting parts in which damage to any component destroys are severly damages the system's function. The later definitions added the concept that the IC system also must have arisen from several steps, none of which underwent selection. Behe was basically saying that an IC system which had to have arisen from a large number of selectively neutral transitions was less likely to evolve. Well, duh: That's basically *defining* an IC system as one that's unlikely to evolve. The problem with the additional criteria was that it now required Behe to demonstrate which transitions involved neutral steps and whether they were so improbable that they couldn't have happened in nature. That is something that Behe has *never* accomplished.

To summarize: Behe's original formulation of IC identified a subset of biological systems that met a *functional* criteria. It was his job to connect that to the relative evolvabilty of that class of systems. With the second verion of his IC definition he entangled a functional criterion with criteria related to historical pathways. Unfortunately, by considering evolvability in his newer criteria he's begging the question.

Strange indeed, then, that the numbers seem to be coming from the IDers

It should be obvious that biologists presents such pathways not numbers. And that IDers don't as they always discuss a priori probabilities, probabilities which are besides the point as Unsympathetic Reader explains.

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Quotations

The old argument of design in nature, as given by Paley, which formerlyseemed to me to be so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.

Charles Darwin (c1880)Although I am fully convinced of the truth of the views given in this volume, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory.

Charles Darwin (1859)Science reveals where religion conceals. Where religion purports to explain, it actually resorts to tautology. To assert that "God did it" is no more than an admission of ignorance dressed deceitfully as an explanation...

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I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.

The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies raw material only. Natural selection directs the course of evolutionary change.

Rudyard Kipling asked how the leopard got its spots, the rhino its wrinkled skin. He called his answers "just-so stories." When evolutionists try to explain form and behavior, they also tell just-so stories—and the agent is natural selection. Virtuosity in invention replaces testability as the criterion for acceptance.

The first commandment for all versions of NOMA might be summarized by stating: "Thou shalt not mix the magisteria by claiming that God directly ordains important events in the history of nature by special interference knowable only through revelation and not accessible to science." In common parlance, we refer to such special interference as "miracle"—operationally defined as a unique and temporary suspension of natural law to reorder the facts of nature by divine fiat.

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My own view is that conclusions about the evolution of human behavior should be based on research at least as rigorous as that used in studying nonhuman animals. And if you read the animal behavior journals, you'll see that this requirement sets the bar pretty high, so that many assertions about evolutionary psychology sink without a trace.

Jerry Coyne
Why Evolution Is TrueI once made the remark that two things disappeared in 1990: one was communism, the other was biochemistry and that only one of them should be allowed to come back.

Sydney Brenner
TIBS Dec. 2000
It is naïve to think that if a species' environment changes the species must adapt or else become extinct.... Just as a changed environment need not set in motion selection for new adaptations, new adaptations may evolve in an unchanging environment if new mutations arise that are superior to any pre-existing variations

Douglas Futuyma
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Francis Crick
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Sydney Brenner
An atheist before Darwin could have said, following Hume: 'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist

Richard Dawkins
Another curious aspect of the theory of evolution is that everybody thinks he understand it. I mean philosophers, social scientists, and so on. While in fact very few people understand it, actually as it stands, even as it stood when Darwin expressed it, and even less as we now may be able to understand it in biology.

Jacques Monod
The false view of evolution as a process of global optimizing has been applied literally by engineers who, taken in by a mistaken metaphor, have attempted to find globally optimal solutions to design problems by writing programs that model evolution by natural selection.